Stackless issue python#283: Concentrate platform code in slp_transfer.c

Add a changelog entry.

Author: akruis

Stackless/changelog.txt

@@ -9,6 +9,11 @@ What's New in Stackless 3.X.X?
 
 *Release date: 20XX-XX-XX*
 
+- https://github.com/stackless-dev/stackless/issues/283
+  Platform dependent source code is now completely in "Stackless/platf". The
+  API header files do not depend on the platform any longer and the header
+  "pycore_slp_platformselect.h" does not exist any more.
+
 - https://github.com/stackless-dev/stackless/issues/281
   Per tasklet profiling and tracing state information is now stored in the
   tasklet itself. This increases the size of tasklet objects a bit, but saves a